1. Field of the Invention
The present invention relates to particulates, particularly ultrafine particulates of titanium oxide obtained by a vapor phase process and a production process thereof. Moreover, the present invention relates to particulates, particularly ultrafine particulates of titanium oxide obtained from starting material of titanium tetrachloride, which particulates contain less aggregated particles and have excellent dispersibility. The present invention also relates to a production process of producing such particulates.
2. Description of the Related Art
Particulates, particularly ultrafine particulates of titanium oxide have very wide application areas in the industrial field and their diversified uses include an ultraviolet-shielding material, an additive to silicone rubber, a photocatalyst and the like. The “titanium oxide” is referred to as “titanium dioxide” in Japanese Industrial Standard (JIS) but the term “titanium oxide” is used as a common name. Accordingly, this simple term “titanium oxide” is hereinafter used in the present invention.
The importance of titanium oxide is increasing in the use for shielding an ultraviolet ray, for example, in the field of cosmetics, clothing and the like. As a shielding material, ultrafine particulates of titanium oxide are being used in many cases because of its high safety. For the shielding, two functions of absorbing and scattering the ultraviolet rays are necessary. The ultrafine particulates of titanium oxide have both of these two functions.
The titanium oxide has a property of absorbing ultraviolet rays at a wavelength of about 400 nm or less to excite electrons. When the electrons and the holes generated reach the surface of particulates, they combine with oxygen or water to generate various radical species. The radical species have an action of decomposing organic materials and therefore, in the case of using titanium oxide in cosmetics and the like, the ultrafine particulates of titanium oxide are generally surface-treated in advance. The fine particulates of titanium oxide are also used for making use of the photocatalytic reaction resulting from photoexcitation of titanium oxide. Furthermore, where titanium oxide is used for scattering ultraviolet rays, ultrafine particulates of titanium oxide having a primary particle size of about 80 nm are used. Although ultrafine particulates in general are not strictly defined with respect to the primary particle size, fine particles having a primary particle size of about 0.1 μm or less are usually called ultrafine particles (particulates).
The titanium oxide is generally produced using a liquid phase process where titanium tetrachloride or titanyl sulfate as a starting material is hydrolyzed in a hydrophilic solvent or a vapor phase process where a volatile starting material such as titanium tetrachloride is vaporized and then reacted in the gas state with an oxidizing gas such as oxygen or steam at a high temperature. For example, JP-A-1-145307 discloses a method of producing ultrafine spherical particulates of metal oxide by setting the flow rate of either one of a volatile metal oxide and steam at 5 m/sec or more.
In general, the titanium oxide powder produced by the liquid phase process disadvantageously undergoes heavy aggregation. Accordingly, on use of titanium oxide in cosmetics and the like, the titanium oxide must be strongly cracked or pulverized and as a result, there arise problems such as mingling of abraded materials attributable to the pulverization treatment or the like, non-uniform distribution of the particle size, or bad touch feeling.
In the case of titanium oxide produced by the vapor phase process, the same problems as in the production by the liquid phase process will arise. That is, although ultrafine particulates of titanium oxide may be obtained by the conventional vapor phase process, only particulates of titanium oxide which have underwent grain growth can be obtained, so that for obtaining ultrafine particulates of titanium oxide, the titanium oxide must be strongly cracked or pulverized.